The direct observation of gravitational waves (GWs) produced during the relativistic collision of two compact objects, offers exciting new opportunities for the study of the nature of the colliding bodies. In the near future, following the recent LIGO detections [1]}-[2]}, GW astronomy will provide us a new understanding of the gravitational interaction and astrophysics in extreme-gravity conditions. The recent observations do not yet probe the detailed structure of spacetime beyond the photon sphere, however one expects that the strong gravity regime will in the next years come to our understanding with future GW observations. In particular, the expectation is to precisely detect the ringdown phase, which is governed by a series of damped oscillatory modes at early times, named quasinormal modes (QNMs) [3]}-[4]}, and may potentially contain unexpected anomalies due to new physics at late times [5]}.
